This macro creates an Xbar-S chart that is robust to the presence of diffuse and localized disturbances; either a shift affecting specific subgroups or extreme individual observations within a subgroup.
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The use of this version of the macro is limited to subgroups of size 5, 7, 9, 11, 13, or 15. As to the number of subgroups, the macro assumes the user will have either 20, 30, or 40. If one of these conditions is not met, the macro will produce an error message and close. This is because the unbiasing constants needed to calculate a robust estimate of sigma are currently only available for these values.
Suppose your observations are in C1 and the subgroup IDs are in C2. To run the macro, chooseand type:
%ROBUSTCHART C1 C2
Click Submit Commands.
Because Phase I estimates are assumed to be calculated from a process that is stable and under statistical control, robust parameter estimates need to be computed in the presence of special-cause variation. For more technical details about the algorithm, refer to the references below:
Tatum, L.G. (1997). "Robust Estimation of the Process Standard Deviation for Control Charts", Technometrics, Vol. 39(2): 127-141.
Schoonhoven, M., Nazir, H.Z., Riaz, M., and Does, R.J.M.M. (2011). "Robust Location Estimators for the Xbar Control Chart", JQT. Vol. 43(4): 363-379.